The Javadoc comment below states that the method "throws IllegalArgumentException if the map is null":
/**
....
* @param map the map to use to transform the objects
* @return the transformer
* @throws IllegalArgumentException if the map is null
*/
public static <I, O> Transformer<I, O> mapTransformer(Map<? super I, ? extends O> map) {
return MapTransformer.mapTransformer(map);
}
However, the method returns a NULL_INSTANCE object instead of throwing IllegalArgumentException when called with null.

Suggested Fixes:
1. Change "@throws IllegalArgumentException if the map is null" and "@return" to "@return NULL_INSTANCE if the map is null".
or
2. Remove the entire "throws IllegalArgumentException if the map is null".